If it says (Gal.) under "Pool Size" then yes, it's gallons. If it says (L) then it's calculating in metric units and you'll need to go into the settings menu and change to imperial units. The Bleach Strength is the % of sodium hypochlorite in your bleach bottle (usually listed under ingredients or in the "warning" area of the label). Then you enter EITHER the PPM increase you're trying to obtain or the amount of bleach you've added. Then the calc with give you the OTHER calculation (the one you DIDN'T enter) when you hit the "calculate" button.